Junos OSPF vs IS-IS Overload

Juniper, Network Engineering

On Junos, both OSPF and IS-IS have an overload setting. Although they perform a similar function, they achieve this through different methods. This leads to slightly different results when applying the overload setting.

Note: Cisco does not have an overload command for OSPF. Instead, it has the ‘max-metric router-lsa’, which does a similar job.

Why Use Overload

The purpose of the overload setting is to push traffic away from the router. There are a couple of reasons for this:

– During a planned maintenance window where changes on the router might impact traffic passing through it

– The router can be configured to apply the overload setting when the router starts, and automatically remove it after a set amount of time. This is common in MPLS networks, where the IGP may establish and attract traffic through the router before the other protocols are ready, causing traffic to black hole.

How Does The Overload Function Differ Between OSPF and IS-IS

On Junos, the OSPF overload configuration sets the metric for the non-loopback interfaces to the maximum metric of 65535.

The IS-IS protocol, on the other hand, has an actual overload bit that the protocol can set. This indicates to other IS-IS routers that they should not use the router with the overload bit set for transit traffic. It can still be used to reach networks directly connected to the router.

Lab Environment

The lab is set up as per the below diagram. These were all built as logical systems on top of a vJunos Router.

The differences when using logical systems are:

– Configuration is moved down the hierarchy to sit under the relevant logical system

– Operational show commands need to specify the logical system, which you won’t need to do when not using logical systems.

If you are configuring this without logical systems, the commands are all the same, just without the logical-system portion.

  • Each router, R1 through R7, is a logical system.
  • Logical tunnel interfaces are used between the logical systems.
  • With the IS-IS protocol, family iso is configured on the interfaces, and an ISO address is configured under the loopback.
  • Each logical system has its own loopback interface logical unit.
  • For OSPF, the router-id is taken from the loopback interface in each logical system.

Example 1 – IS-IS Overload Test

To test the overload function, we will set the overload bit on the R3 router. The output below is collated to show the difference before and after the overload setting is configured.

All output is taken from the perspective of R1, specifically, we are interested in the output for traffic to R3 and R7.

Configuration

All seven routers have a simple IS-IS configuration, similar to what is shown for the R1 logical system.

lab@vRouter-1> show configuration logical-systems R1 protocols isis 
interface all;
interface lo0.1 {
    passive;
}
level 1 disable;

lab@vRouter-1>

We can set the overload for IS-IS on the R3 logical system.

lab@vRouter-1> edit   
Entering configuration mode

[edit]
lab@vRouter-1# set logical-systems R3 protocols isis overload 

[edit]
lab@vRouter-1# commit and-quit 
commit complete
Exiting configuration mode

lab@vRouter-1>

Routing Output

First, let’s check the routing table beforehand.

lab@vRouter-1> show route logical-system R1 192.168.0.3 

inet.0: 17 destinations, 17 routes (17 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.3/32     *[IS-IS/18] 04:37:03, metric 10
                    >  to 172.16.13.2 via lt-0/0/0.3

lab@vRouter-1> show route logical-system R1 192.168.0.7    

inet.0: 17 destinations, 17 routes (17 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.7/32     *[IS-IS/18] 00:01:49, metric 20
                    >  to 172.16.13.2 via lt-0/0/0.3

After the overload setting is configured, we see the following.

lab@vRouter-1> show route logical-system R1 192.168.0.3 

inet.0: 16 destinations, 16 routes (16 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.3/32     *[IS-IS/18] 04:43:05, metric 10
                    >  to 172.16.13.2 via lt-0/0/0.3

lab@vRouter-1> show route logical-system R1 192.168.0.7    

lab@vRouter-1>

The route to R3’s loopback address remains the same. The route for R7’s loopback has disappeared though.

Traceroute Output

The traceroutes without overload are as follows.

lab@vRouter-1> traceroute logical-system R1 192.168.0.3 
traceroute to 192.168.0.3 (192.168.0.3), 30 hops max, 52 byte packets
 1  192.168.0.3 (192.168.0.3)  5.844 ms  5.102 ms  4.571 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.7    
traceroute to 192.168.0.7 (192.168.0.7), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  5.284 ms  4.959 ms  3.495 ms
 2  192.168.0.7 (192.168.0.7)  11.175 ms  5.838 ms  4.975 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.5    
traceroute to 192.168.0.5 (192.168.0.5), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  3.535 ms  2.525 ms  3.645 ms
 2  192.168.0.5 (192.168.0.5)  7.043 ms  4.678 ms  3.989 ms

The traffic for R1 to R3’s loopback is R1 – R3.

The traffic for R1 to R7’s loopback is R1 – R3 – R7.

After the overload bit is set, it changes to the following.

lab@vRouter-1> traceroute logical-system R1 192.168.0.3 
traceroute to 192.168.0.3 (192.168.0.3), 30 hops max, 52 byte packets
 1  192.168.0.3 (192.168.0.3)  5.775 ms  4.737 ms  7.622 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.7    
traceroute to 192.168.0.7 (192.168.0.7), 30 hops max, 52 byte packets
traceroute: sendto: No route to host
 1 traceroute: wrote 192.168.0.7 52 chars, ret=-1
 *traceroute: sendto: No route to host
traceroute: wrote 192.168.0.7 52 chars, ret=-1
^C
lab@vRouter-1>

The path from R1 to R3’s loopback is still R1 – R3.

Since there is no route for R7’s loopback, the traceroute fails.

We can also check the traceroute to R5’s loopback.

lab@vRouter-1> traceroute logical-system R1 192.168.0.5    
traceroute to 192.168.0.5 (192.168.0.5), 30 hops max, 52 byte packets
 1  172.16.12.2 (172.16.12.2)  3.131 ms  2.907 ms  4.278 ms
 2  172.16.24.2 (172.16.24.2)  3.143 ms  3.994 ms  3.418 ms
 3  172.16.46.2 (172.16.46.2)  4.939 ms  3.573 ms  2.419 ms
 4  192.168.0.5 (192.168.0.5)  4.724 ms  5.566 ms  5.126 ms

lab@vRouter-1>

The path from R1 to R5’s loopback address is now R1 – R2 – R4 – R6 – R5.

Next, we look at the differences in the IS-IS database.

Protocol Database Output

Before overload is set.

lab@vRouter-1> show isis database logical-system R1 
IS-IS level 1 link-state database:
  0 LSPs

IS-IS level 2 link-state database:
LSP ID                      Sequence Checksum Lifetime Attributes
vRouter-1-R1.00-00              0x18   0x6d76     1075 L1 L2
vRouter-1-R2.00-00              0x18   0x2b7f      699 L1 L2
vRouter-1-R2.02-00              0x16   0xc6a0     1048 L1 L2
vRouter-1-R3.00-00              0x1b   0x76aa     1049 L1 L2
vRouter-1-R3.02-00              0x16   0xee75     1141 L1 L2
vRouter-1-R4.00-00              0x19   0x797f     1002 L1 L2
vRouter-1-R4.02-00              0x16   0xa3b9      585 L1 L2
vRouter-1-R4.03-00              0x16   0x5608     1132 L1 L2
vRouter-1-R5.00-00              0x18   0xdbe1      852 L1 L2
vRouter-1-R5.02-00              0x15   0xcd8d      591 L1 L2
vRouter-1-R6.00-00              0x18   0x7f07      892 L1 L2
vRouter-1-R6.02-00              0x16   0x3a1b      831 L1 L2
vRouter-1-R6.03-00              0x16   0x79d8     1026 L1 L2
vRouter-1-R7.00-00              0x16   0x8a73      532 L1 L2
vRouter-1-R7.02-00              0x15   0x1e37      716 L1 L2
  15 LSPs

lab@vRouter-1>

After overload is set, the overloaded router is identified in the database.

lab@vRouter-1> show isis database logical-system R1 
IS-IS level 1 link-state database:
  0 LSPs

IS-IS level 2 link-state database:
LSP ID                      Sequence Checksum Lifetime Attributes
vRouter-1-R1.00-00              0x18   0x6d76      712 L1 L2
vRouter-1-R2.00-00              0x19   0x2980     1028 L1 L2
vRouter-1-R2.02-00              0x16   0xc6a0      685 L1 L2
vRouter-1-R3.00-00              0x1c   0x9586     1069 L1 L2 Overload
vRouter-1-R3.02-00              0x16   0xee75      779 L1 L2
vRouter-1-R4.00-00              0x19   0x797f      639 L1 L2
vRouter-1-R4.02-00              0x17   0xa1ba     1051 L1 L2
vRouter-1-R4.03-00              0x16   0x5608      770 L1 L2
vRouter-1-R5.00-00              0x18   0xdbe1      490 L1 L2
vRouter-1-R5.02-00              0x16   0xcb8e      969 L1 L2
vRouter-1-R6.00-00              0x18   0x7f07      529 L1 L2
vRouter-1-R6.02-00              0x17   0x381c     1171 L1 L2
vRouter-1-R6.03-00              0x16   0x79d8      664 L1 L2
vRouter-1-R7.00-00              0x17   0x8874      961 L1 L2
vRouter-1-R7.02-00              0x16   0x1c38     1070 L1 L2
  15 LSPs

lab@vRouter-1>

And the detailed LSP output for the R3 and R7 LSPs.

lab@vRouter-1> show isis database vRouter-1-R3.00-00 logical-system R1 detail       
IS-IS level 1 link-state database:

IS-IS level 2 link-state database:

vRouter-1-R3.00-00 Sequence: 0x1b, Checksum: 0x76aa, Lifetime: 929 secs
   IS neighbor: vRouter-1-R3.02               Metric:       10
   IS neighbor: vRouter-1-R4.02               Metric:       10
   IS neighbor: vRouter-1-R5.02               Metric:       10
   IS neighbor: vRouter-1-R7.02               Metric:       10
   IP prefix: 172.16.13.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.34.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.35.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.37.0/24                  Metric:       10 Internal Up
   IP prefix: 192.168.0.3/32                  Metric:        0 Internal Up

lab@vRouter-1> show isis database vRouter-1-R7.00-00 logical-system R1 detail    
IS-IS level 1 link-state database:

IS-IS level 2 link-state database:

vRouter-1-R7.00-00 Sequence: 0x17, Checksum: 0x8874, Lifetime: 1190 secs
   IS neighbor: vRouter-1-R7.02               Metric:       10
   IP prefix: 172.16.37.0/24                  Metric:       10 Internal Up
   IP prefix: 192.168.0.7/32                  Metric:        0 Internal Up

lab@vRouter-1>

After overload is set.

lab@vRouter-1> show isis database vRouter-1-R3.00-00 logical-system R1 detail    
IS-IS level 1 link-state database:

IS-IS level 2 link-state database:

vRouter-1-R3.00-00 Sequence: 0x1c, Checksum: 0x9586, Lifetime: 999 secs
   IS neighbor: vRouter-1-R3.02               Metric:       10
   IS neighbor: vRouter-1-R4.02               Metric:       10
   IS neighbor: vRouter-1-R5.02               Metric:       10
   IS neighbor: vRouter-1-R7.02               Metric:       10
   IP prefix: 172.16.13.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.34.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.35.0/24                  Metric:       10 Internal Up
   IP prefix: 172.16.37.0/24                  Metric:       10 Internal Up
   IP prefix: 192.168.0.3/32                  Metric:        0 Internal Up

lab@vRouter-1> show isis database vRouter-1-R7.00-00 logical-system R1 detail    
IS-IS level 1 link-state database:

IS-IS level 2 link-state database:

vRouter-1-R7.00-00 Sequence: 0x17, Checksum: 0x8874, Lifetime: 885 secs
   IS neighbor: vRouter-1-R7.02               Metric:       10
   IP prefix: 172.16.37.0/24                  Metric:       10 Internal Up
   IP prefix: 192.168.0.7/32                  Metric:        0 Internal Up

lab@vRouter-1>

The individual LSP entries are basically the same. Since the link-state database needs to be consistent across all the routers, R1 still has the R7 LSP in its IS-IS database.

Even though the R7 loopback is only reachable via R3, R1 does not install a route for it, as that would have to transit through R3. This is prevented by the overload bit being set.

From R1’s perspective, it will use R3 to reach any network directly connected to R3.

This means, for example, it will still use R3 to reach addresses on the R3 to R5 link-net.

lab@vRouter-1> traceroute logical-system R1 172.16.35.1                             
traceroute to 172.16.35.1 (172.16.35.1), 30 hops max, 52 byte packets
 1  172.16.35.1 (172.16.35.1)  4.379 ms  4.502 ms  4.594 ms

lab@vRouter-1> traceroute logical-system R1 172.16.35.2    
traceroute to 172.16.35.2 (172.16.35.2), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  2.811 ms  4.459 ms  4.059 ms
 2  172.16.35.2 (172.16.35.2)  4.771 ms  7.331 ms  6.112 ms

Example 2 – OSPF Overload Test

The logical systems are all configured similarly to R1. The only difference is the loopback unit specified.

lab@vRouter-1> show configuration logical-systems R1 protocols ospf 
area 0.0.0.0 {
    interface lo0.1 {
        passive;
    }
    interface all;
}
reference-bandwidth 10g;

lab@vRouter-1>

The overload bit can be set on the R3 logical system as per the below config.

lab@vRouter-1> edit 
Entering configuration mode

[edit]
lab@vRouter-1# set logical-systems R3 protocols ospf overload 

[edit]
lab@vRouter-1# commit and-quit 
commit complete
Exiting configuration mode

lab@vRouter-1>

Routing Output

The routing output without overload set on R3 is shown below.

lab@vRouter-1> show route logical-system R1 192.168.0.3 

inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.3/32     *[OSPF/10] 00:00:42, metric 1
                    >  to 172.16.13.2 via lt-0/0/0.3

lab@vRouter-1> show route logical-system R1 192.168.0.7    

inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.7/32     *[OSPF/10] 00:00:49, metric 2
                    >  to 172.16.13.2 via lt-0/0/0.3

After overload is set, the routes now look as follows.

lab@vRouter-1> show route logical-system R1 192.168.0.3 

inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.3/32     *[OSPF/10] 00:06:19, metric 1
                    >  to 172.16.13.2 via lt-0/0/0.3

lab@vRouter-1> show route logical-system R1 192.168.0.7    

inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.0.7/32     *[OSPF/10] 00:00:48, metric 65536
                    >  to 172.16.13.2 via lt-0/0/0.3

lab@vRouter-1>

Notice how the metric for the 192.168.0.7 network has changed to 65536. This differs from IS-IS, where the route wasn’t visible because R3 couldn’t be used as a transit router.

Next, let’s compare traceroutes.

Traceroute Output

Without the overload bit set, the traceroutes look as you’d expect them to.

lab@vRouter-1> traceroute logical-system R1 192.168.0.3    
traceroute to 192.168.0.3 (192.168.0.3), 30 hops max, 52 byte packets
 1  192.168.0.3 (192.168.0.3)  4.770 ms  4.721 ms  5.888 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.7    
traceroute to 192.168.0.7 (192.168.0.7), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  2.995 ms  3.071 ms  3.538 ms
 2  192.168.0.7 (192.168.0.7)  9.019 ms  3.893 ms  4.240 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.5    
traceroute to 192.168.0.5 (192.168.0.5), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  2.760 ms  2.827 ms  2.117 ms
 2  192.168.0.5 (192.168.0.5)  5.035 ms  4.889 ms  3.742 ms

lab@vRouter-1>

Traffic from R1 to R3, R7, or R5 takes the shortest path via R3.

And again, after overload is set.

lab@vRouter-1> traceroute logical-system R1 192.168.0.3 
traceroute to 192.168.0.3 (192.168.0.3), 30 hops max, 52 byte packets
 1  192.168.0.3 (192.168.0.3)  5.959 ms  3.973 ms  293.415 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.7    
traceroute to 192.168.0.7 (192.168.0.7), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  3.061 ms  2.577 ms  3.291 ms
 2  192.168.0.7 (192.168.0.7)  4.080 ms  4.836 ms  4.696 ms

lab@vRouter-1> traceroute logical-system R1 192.168.0.5    
traceroute to 192.168.0.5 (192.168.0.5), 30 hops max, 52 byte packets
 1  172.16.12.2 (172.16.12.2)  3.511 ms  2.899 ms  4.607 ms
 2  172.16.24.2 (172.16.24.2)  5.984 ms  4.295 ms  6.251 ms
 3  172.16.46.2 (172.16.46.2)  3.427 ms  3.305 ms  3.711 ms
 4  192.168.0.5 (192.168.0.5)  4.285 ms  4.394 ms  4.372 ms

lab@vRouter-1>

Traffic from R1 to the loopbacks of R3 and R7 still passes via R3. This is to be expected, no matter how big the metric, R3 is still the only path to reach R7’s loopback.

The path to R5 has changed to be R1 – R2 – R4 – R6 – R5.

Testing a traceroute from R1 to the link-net for R3 to R5 shows another difference.

Before:

lab@vRouter-1> traceroute logical-system R1 172.16.35.1     
traceroute to 172.16.35.1 (172.16.35.1), 30 hops max, 52 byte packets
 1  172.16.35.1 (172.16.35.1)  5.654 ms  6.311 ms  4.525 ms

lab@vRouter-1> traceroute logical-system R1 172.16.35.2    
traceroute to 172.16.35.2 (172.16.35.2), 30 hops max, 52 byte packets
 1  172.16.13.2 (172.16.13.2)  62.105 ms  3.541 ms  3.216 ms
 2  172.16.35.2 (172.16.35.2)  5.094 ms  5.641 ms  4.922 ms

lab@vRouter-1>

After overload is set:

lab@vRouter-1> traceroute logical-system R1 172.16.35.1    
traceroute to 172.16.35.1 (172.16.35.1), 30 hops max, 52 byte packets
 1  172.16.12.2 (172.16.12.2)  2.311 ms  2.847 ms  3.240 ms
 2  172.16.24.2 (172.16.24.2)  4.023 ms  5.114 ms  4.003 ms
 3  172.16.46.2 (172.16.46.2)  4.140 ms  3.909 ms  3.349 ms
 4  172.16.56.1 (172.16.56.1)  3.795 ms  4.958 ms  5.242 ms
 5  172.16.35.1 (172.16.35.1)  6.858 ms  4.748 ms  4.805 ms

lab@vRouter-1> traceroute logical-system R1 172.16.35.2    
traceroute to 172.16.35.2 (172.16.35.2), 30 hops max, 52 byte packets
 1  172.16.12.2 (172.16.12.2)  3.601 ms  2.755 ms  4.080 ms
 2  172.16.24.2 (172.16.24.2)  5.092 ms  5.013 ms  3.091 ms
 3  172.16.46.2 (172.16.46.2)  5.016 ms  3.620 ms  7.032 ms
 4  172.16.35.2 (172.16.35.2)  4.737 ms  6.087 ms  4.342 ms

lab@vRouter-1>

Setting overload on R3 changes the metric on all the interfaces attached to R3. This causes R1 to prefer to send traffic the long way around the network because it has a lower overall cost.

This differs from IS-IS, where the metrics were kept the same, but R3 could not be used as a transit router.

Protocol Database Output

The OSPF database doesn’t look much different before and after the overload setting is configured.

Before:

lab@vRouter-1> show ospf database logical-system R1        

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *192.168.0.1      192.168.0.1      0x80000003   243  0x22 0x1a5   60
Router   192.168.0.2      192.168.0.2      0x80000003   249  0x22 0x1a72  60
Router   192.168.0.3      192.168.0.3      0x80000005   239  0x22 0xd5f1  84
Router   192.168.0.4      192.168.0.4      0x80000004   245  0x22 0xb8b6  72
Router   192.168.0.5      192.168.0.5      0x80000003   257  0x22 0xf61e  60
Router   192.168.0.6      192.168.0.6      0x80000003   256  0x22 0x13e7  60
Router   192.168.0.7      192.168.0.7      0x80000002   267  0x22 0x20e8  48
Network  172.16.12.2      192.168.0.2      0x80000001   262  0x22 0x46e7  32
Network  172.16.13.2      192.168.0.3      0x80000001   244  0x22 0x3feb  32
Network  172.16.24.2      192.168.0.4      0x80000001   249  0x22 0xd745  32
Network  172.16.34.2      192.168.0.4      0x80000001   245  0x22 0x779a  32
Network  172.16.35.2      192.168.0.5      0x80000001   257  0x22 0x709e  32
Network  172.16.37.2      192.168.0.7      0x80000001   267  0x22 0x62a6  32
Network  172.16.46.2      192.168.0.6      0x80000001   256  0x22 0x9f7   32
Network  172.16.56.2      192.168.0.6      0x80000001   264  0x22 0xa84d  32

After:

lab@vRouter-1> show ospf database logical-system R1                              

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *192.168.0.1      192.168.0.1      0x80000003   563  0x22 0x1a5   60
Router   192.168.0.2      192.168.0.2      0x80000003   569  0x22 0x1a72  60
Router   192.168.0.3      192.168.0.3      0x80000006   210  0x22 0x13b7  84
Router   192.168.0.4      192.168.0.4      0x80000004   565  0x22 0xb8b6  72
Router   192.168.0.5      192.168.0.5      0x80000003   577  0x22 0xf61e  60
Router   192.168.0.6      192.168.0.6      0x80000003   576  0x22 0x13e7  60
Router   192.168.0.7      192.168.0.7      0x80000002   587  0x22 0x20e8  48
Network  172.16.12.2      192.168.0.2      0x80000001   582  0x22 0x46e7  32
Network  172.16.13.2      192.168.0.3      0x80000001   564  0x22 0x3feb  32
Network  172.16.24.2      192.168.0.4      0x80000001   569  0x22 0xd745  32
Network  172.16.34.2      192.168.0.4      0x80000001   565  0x22 0x779a  32
Network  172.16.35.2      192.168.0.5      0x80000001   577  0x22 0x709e  32
Network  172.16.37.2      192.168.0.7      0x80000001   587  0x22 0x62a6  32
Network  172.16.46.2      192.168.0.6      0x80000001   576  0x22 0x9f7   32
Network  172.16.56.2      192.168.0.6      0x80000001   584  0x22 0xa84d  32

The individual LSAs do show some differences, particularly the metrics that are set.

Before:

lab@vRouter-1> show ospf database logical-system R1 lsa-id 192.168.0.3 detail 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router   192.168.0.3      192.168.0.3      0x80000005   256  0x22 0xd5f1  84
  bits 0x0, link count 5
  id 192.168.0.3, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 172.16.37.2, data 172.16.37.1, Type Transit (2)
    Topology count: 0, Default metric: 1
  id 172.16.13.2, data 172.16.13.2, Type Transit (2)
    Topology count: 0, Default metric: 1
  id 172.16.34.2, data 172.16.34.1, Type Transit (2)
    Topology count: 0, Default metric: 1
  id 172.16.35.2, data 172.16.35.1, Type Transit (2)
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID: 172.16.35.2
      Metric: 1, Bidirectional
    Type: Transit, Node ID: 172.16.34.2
      Metric: 1, Bidirectional
    Type: Transit, Node ID: 172.16.13.2
      Metric: 1, Bidirectional
    Type: Transit, Node ID: 172.16.37.2
      Metric: 1, Bidirectional          

lab@vRouter-1> show ospf database logical-system R1 lsa-id 192.168.0.7 detail    

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router   192.168.0.7      192.168.0.7      0x80000002   289  0x22 0x20e8  48
  bits 0x0, link count 2
  id 192.168.0.7, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 172.16.37.2, data 172.16.37.2, Type Transit (2)
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID: 172.16.37.2
      Metric: 1, Bidirectional

lab@vRouter-1>

After:

lab@vRouter-1> show ospf database logical-system R1 lsa-id 192.168.0.3 detail 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router   192.168.0.3      192.168.0.3      0x80000006   225  0x22 0x13b7  84
  bits 0x0, link count 5
  id 192.168.0.3, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 172.16.37.2, data 172.16.37.1, Type Transit (2)
    Topology count: 0, Default metric: 65535
  id 172.16.13.2, data 172.16.13.2, Type Transit (2)
    Topology count: 0, Default metric: 65535
  id 172.16.34.2, data 172.16.34.1, Type Transit (2)
    Topology count: 0, Default metric: 65535
  id 172.16.35.2, data 172.16.35.1, Type Transit (2)
    Topology count: 0, Default metric: 65535
  Topology default (ID 0)
    Type: Transit, Node ID: 172.16.35.2
      Metric: 65535, Bidirectional
    Type: Transit, Node ID: 172.16.34.2
      Metric: 65535, Bidirectional
    Type: Transit, Node ID: 172.16.13.2
      Metric: 65535, Bidirectional
    Type: Transit, Node ID: 172.16.37.2
      Metric: 65535, Bidirectional      

lab@vRouter-1> show ospf database logical-system R1 lsa-id 192.168.0.7 detail    

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router   192.168.0.7      192.168.0.7      0x80000002   609  0x22 0x20e8  48
  bits 0x0, link count 2
  id 192.168.0.7, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 172.16.37.2, data 172.16.37.2, Type Transit (2)
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID: 172.16.37.2
      Metric: 1, Bidirectional

lab@vRouter-1>

Summary

The key takeaways are:

  • IS-IS overload does not allow any transit traffic through a node, only traffic to networks local to the overloaded router.
  • IS-IS overload does not change the metrics in the LSPs.
  • OSPF overload sets the metrics for the overloaded router to the maximum cost, which forces traffic to take other lower-cost paths if they are available.
  • OSPF overload does allow transit traffic through the overloaded router if it is either the only or the best cost path to reach a destination.
  • Other vendors may not have an OSPF overload setting but may have a max-metric setting instead.

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