Two-way redistribution: Why it's not broken?

[michael.luo]

This is just for the sake of discussion.

I have a border router that is running EIGRP and eBGP.  I'm doing two-way redistribution like below:

router eigrp 1

 redistribute bgp 64801 metric 1500 1 255 1 1500

router bgp 64801

 redistribute eigrp 1

When using the command "show ip bgp neighbor a.b.c.d advertised-routes", I see that the routes learned from eBGP neighbor (foreign routes) were not being advertised.  This is a normal and expected behavior.  But the dumb question is:

1) Since we're redistributing BGP routes into EIGRP, EIGRP has those foreign routes.

2) At the meantime, we're redistributing EIGRP routes into BGP (including the foreign routes).  Why BGP is not advertising the foreign routes out to eBGP peer?

Again, just for the sake of discussion.  Thanks!

[michael.luo]

Here are the output from the router.  20.20.20.0/24 is learned from eBGP.  The question is - why 20.20.20.0/24 was not being advertised back to eBGP peers?

R2#sh ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(172.255.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.255.1.0/30, 1 successors, FD is 281600
        via Connected, Ethernet0/0
P 20.255.1.0/30, 1 successors, FD is 1706752, tag is 64002
        via Redistributed (1706752/0)
P 20.20.20.0/24, 1 successors, FD is 1706752, tag is 64002
        via Redistributed (1706752/0)
P 172.255.1.0/30, 1 successors, FD is 281600
        via Connected, Ethernet1/1
P 10.10.10.0/24, 1 successors, FD is 409600
        via 10.255.1.2 (409600/128256), Ethernet0/0

R2#

R2#sh ip bgp
BGP table version is 15, local router ID is 172.255.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  10.10.10.0/24    10.255.1.2          409600         32768 ?
 *>  10.255.1.0/30    0.0.0.0                  0         32768 ?
 *>  20.20.20.0/24    172.255.1.2         409600             0 64002 ?
 *>  20.255.1.0/30    172.255.1.2              0             0 64002 ?
 *>  172.255.1.0/30   0.0.0.0                  0         32768 ?
 *                    172.255.1.2              0             0 64002 ?

R2#sh ip bgp nei 172.255.1.2 advertised-routes
BGP table version is 15, local router ID is 172.255.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  10.10.10.0/24    10.255.1.2          409600         32768 ?
 *>  10.255.1.0/30    0.0.0.0                  0         32768 ?
 *>  172.255.1.0/30   0.0.0.0                  0         32768 ?

Total number of prefixes 3
R2#

[Rolf Fischer · Accepted solution]

Hi,

routes are only redistributed when they are in the routing-table. The route is learned via eBGP and a routing-table entry is created with an AD of 20. This route is then redistributed into EIGRP but no EIGRP route will be installed locally because EIGRP has a higher AD than eBGP and, even more important, self-orinated external routes are ignored for local route calculations. So a redistribution back into BGP cannot take place even though it is in the EIGRP topology table - it is required to be present in the routing table.

HTH
Rolf