Static Route with AD 250 - Redistribution in EIGRP - Behavior

[bravealikhan]

Hi,

RTR1, RTR2, and SW1 are physically connected, and there is an EIGRP neighborship established among them. RTR1 is the primary router for SW1, and RTR2 is the backup.

I set up static routes with track 500 on both RTR1 and RTR2 so that if RTR1 goes down, RTR2 should advertise the default route to SW1.

Example configuration:
On RTR1:

ip route 0.0.0.0 0.0.0.0 [WAN IP] tag 101010 track 500

On RTR2:

ip route 0.0.0.0 0.0.0.0 [WAN IP] 250 tag 101010 track 500

Both routes are being redistributed into EIGRP for SW1's default route.

Issue:
When I set an Administrative Distance (AD) of 250 on the static route of RTR2, I noticed that when RTR1 goes down, RTR2 doesn't take over by advertising the default route to SW1 via redistribution.

However, if I remove the 250 AD value, SW1 immediately switches to the default route via RTR2.

I understand that the AD value of an EIGRP external route is 170. I tried setting this value as well:

ip route 0.0.0.0 0.0.0.0 [WAN IP] 170 tag 101010 track 500

But the route still doesn't get advertised to SW1.

I'd appreciate any guidance.

[David Ruess]

Hello,

Can you provide configurations of all 3 devices and a topology diagram.

-David

[paul driver]

Hello
Advertising a default route in eigrp can be accomplished a couple of ways , the way your describing is via redistribution of a static route, as such the administrative distance you set on that static route has NO bearing when its redistributed into EIGRP that static will then have a external eigrp seed metric of 170 and not the AD distance you set on the static route prior to redistribution .

If you wish to prioritise which default take precedence you can always set a lower/higher delay value on the eigrp interface of the switch facing the the less or preferred rtr path
Edited- also note the reason why the advertisement work when you removed the ad250 value is beforehand RTR2 may have received the redistributed default from RTR1 with a preferred AD of 170

[Giuseppe Larosa]

Hello @paul driver ,

>> also note the reason why the advertisement work when you removed the ad250 value is beforehand RTR2 may have received the redistributed default from RTR1 with a preferred AD of 170

This is true also for the OP @bravealikhan :   there are other ways to make the static route injected into EIGRP by RTR2 less attractive for internal devices for example an higher seed metric  ( that in EIGRP is made of 5 components).

for OP @bravealikhan there are two different activities about default routes:

each of RTR1 and RTR2 can install their own default route if using on RTR2  a standard AD or using a value less then 170.

When redistributing into EIGRP each border router can inject its own default route but you can build a primary/secondary hierarchy (needed for NAT or for having symmetric routing)  by using a different seed metric that provides an higher metric value ( for example you use a greater delay either in the seed metric or at interface level. The second one however needs to be tuned on receiving side of downstream routers because EIGRP advertise each prefix with its metric components ( , cumulative delay, minimum bandwidth on the path , load, realibility MTU value and also hop count).

This is true both for traditional EIGRP and also for named mode where the components of metric have different values and names  and a RIB scaling factor ( default value 128) is used to scale down from 64 bit wide metric of named mode to 32 bit values that can be installed in the IP routing table)

As noted by @paul driver there are other options to generate a default route in EIGRP , special care is needed when using ip eigrp <asn> summary-address 0.0.0.0 0.0.0.0   at interface level for classic EIGRP.,    because it has a default AD of 5 (EIGRP summary route)  in this case adding a floating AD of 200 is recommended in scenarios where the default route comes from OSPF or other IGP.

I'm not sure where in EIGRP named mode you can configure an EIGRP summary address under router mode interface submode or under named routed mode topology submode

. my last note above is about classic EIGRP so it can be old.

Hope to help

Giuseppe

[M02@rt37]

Hello Paul

Yes, in EIGRP named mode, the summary address is configured under the af-interfaces submode, not directly under the topology submode.

--

router eigrp xxx
address-family ipv4 unicast autonomous-system AS_number
af-interface x/x
summary-address 0.0.0.0 0.0.0.0

--

source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/200156-Configure-EIGRP-Named-Mode.html#anc9

[M02@rt37]

Hello @bravealikhan 

As @David Ruess suggested to you, it should be better to share your topology and configurations.

From my point of view, when RTR1 is up, RTR2 still receives the redistributed default route from RTR1 with an AD of 170, making it the preferred route. Since EIGRP does not consider the AD of the static route before redistribution, setting an AD of 250 on RTR2 prevents it from advertising its own default route when RTR1 fails. The reason it work when the AD is removed is that RTR-2 then actively installs the static route and redistributes it into EIGRP...

An other solution would be to use EIGRP route filtering to control when RTR2 advertises the default route. You can configure a route-map with traking so that RTR2 advertises the default only if RTR1 stops announcing it. Another approach is using an offset-list (see command references) to increase the metric of RTR2’s advertised default route, making it less preferred under normal conditions but available as a backup. 

Command references: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_eigrp/command/ire-cr-book/ire-i1.html#wp4196374404

[bravealikhan]

Thank you all for your valuable input! I appreciate it. I’ll create a lab design and share the topology and configuration soon.

[M02@rt37]

Thanks a lot @bravealikhan