Cisco routing protocols

What happens to network traffic that is received on a Cisco router interface and there is no routing protocol servicing the network host.
jlgray48Asked:
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JFrederick29Connect With a Mentor Commented:
It isn't that the router isn't servicing that network, it could learn a route to the 192.168.6.x subnet via another router (learned via EIGRP advertisements).  Running EIGRP on the "networks" tells the router to run EIGRP on those interfaces belonging to the subnets and to also advertise those networks.  If the 192.168.6.x subnet is the destination in the request and it hits the router in question, if the router has a route to that subnet, it will route it appropriately.  If it doesn't have a specific route for that subnet but it has a default route, it will use the default next hop to forward the traffic.  If there is no specific route and no default route, the packet is dropped.  If the 192.168.6.x address is the source in the request, the router doesn't care as routing is based on destination (unless doing policy based routing specifying the source).  The router will need a route to the 192.168.6.x subnet if the return traffic comes back through this router however.  Do a "show ip route 192.168.6.0" on this router to see if it has a specific route to that subnet.
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futurefilesCommented:
not sure what you mean can you clarify?
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jlgray48Author Commented:
Ok, I have a router setup as below

1. cisco 2800 - routing protocol eigrp - networks 192.168.1.x and 192.168.5.x  255.255.255.0

What if another router sends traffic from a 192.168.6.x into a interface on this router. What will happen to the traffic? Will it be dropped since the router isn't routing that network?
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futurefilesCommented:
correct
you will need an acl (access control list) for that network otherwise the packets will be dropped
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futurefilesCommented:
!Create the access list
access-list 101 permit ip192.168.6.0 255.255.255.0 any
!Apply to interface
interface FastEthernet0/1
 ip access-group 101 in
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jlgray48Author Commented:
That is not what I was asking. If the packet shows up on an interface on that router that isn't routing that network, will it drop or will it still route the packet?
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futurefilesCommented:
if its a different network to the router it will always drop it unless you tell it to route it
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jlgray48Author Commented:
SO if my router is only advertising eigrp for the 2 networks it can still accept advertisements from other routers with different AS numbers?

Such as:

Router one:
EIGRP 211
Network 192.168.1.x
Network 192.168.5.x

Router two:
EIGRP 215
Network 192.168.6.x

Obviously router two will advertise the routes to 192.168.6.x but will router one put the routes in it's routing table even though it is a different AS (215 vs 211)
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JFrederick29Commented:
Nope, whole different situation with the different AS's.  Router one and router two won't participate in EIGRP together so routes won't be exchanged.  Router one will use a static route to 192.168.6.x via router two if one exists or use it's default route in an attempt to reach that subnet.
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jlgray48Author Commented:
Ok, I think I have this correct.

Routers will not exchange route advertisements unless they are setup with the same AS number but they will still send the packet out the default route in a best attempt to find a route?

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JFrederick29Commented:
Yes, that is correct.
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MysidiaCommented:
Unless you have applied an ACL denying the traffic, or specified a route map indicating which networks traffic may be allowed from (in a source address), the packet will be accepted and forwarded according to the destination IP in the routing table just like all packets are.

Running a routing protocol or not does not change this behavior.

If there is no route for the 192.168.6.x  network in the routing table, the return traffic will NOT be forwarded back to the interface it came from.

Meaning you will possibly have partial connectivity (host can send traffic but not get return traffic, so it can't establish any TCP connections).


If the router has an interface whose ip address is on the 192.168.6.x   network, then
192.168.6.x  IS in that router's routing table.

However, since you have not enabled the network in the routing protocol, it is likely that the entry for that network is not in the routing table on other routers.

When a router receives a packet destined for a network not in the table, the default route will be utilized.

If there is no default route and no route matching the destination address of a packet, the packet will be dropped.




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jlgray48Author Commented:
So if no routing protocol is setup will a routing table be built with anything other than the directly connected networks?
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MysidiaCommented:
No.  If a router does not participate in any dynamic protocol, there will be
no routes in its table except directly connected networks.

Unless you manually enter static routes or have and enable ODR
(a method of using CDP announcements to populate routing tables).

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jlgray48Author Commented:
In this case if a packet is received on the router it will just send the packet out the default route if one is set or drop the packet if there isn't a default route?
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MysidiaCommented:
Yes, it will.  Unless you have applied traffic filtering or a policy configuration that overrides default behavior and forces that router to do something different.


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lrmooreConnect With a Mentor Commented:
If the router receives a packet from a network that it does not know about (not in route tables) it won't care as it only looks at the destination address, and it will forward on to the destination if the destination next-hop is known, or out the default. However, the return packet will get dropped because the router does not know where to send the return packet. This is all regardless of the routing protocols running or neighbor relationships.
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lrmooreCommented:
Larry.?.?..
It's Les
Thanks!
 <8-}
Sometimes we just get wrapped up in a specific detail and don't look at the whole picture..
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JFrederick29Commented:
Perhaps a point split would have been in order.  Oh well, back to the family...
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JFrederick29Commented:
Thanks lrmoore!  I appreciate it.
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jlgray48Author Commented:
That works for me but how do you split points?

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JFrederick29Commented:
Each comment box has a button that says Accept Multiple Solutions. Click that, and you will see a page that allows you to assign points to any of the comments in the thread. There is a grade box at the bottom of the page.

http://www.experts-exchange.com/help.jsp#hi100
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jlgray48Author Commented:
Thank you Larry
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