punt adjacency and glean adjacency

Unanswered Question
Dec 14th, 2008

Hi every body!

Let say we layer 3 switch with following entry :

10.10.10.0/24 via 10.10.10.2 f0/1

further assume switch does not know the mac for next hop(10.10.10.2)

Will we find punt adjacency because the packet destined to 10.10.10.2 must be forwarded to layer 3 engine or glean adjacency because mac-address is missing?

My understanding is if glean adjacency exists then punt adjacency must exit as well. But can we find both adjacencies( punt,glean adjacency) in adjacency table for the same next-hop ?

thanks a lot!


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Jon Marshall Sun, 12/14/2008 - 15:59

Sarah


I think you know more about this than me now but my understanding is this.


A glean adjacency is one where the host is directly connected but there is no mac-address information for it. But a glean adjacency is more than that. It is an entry for a locally connected subnet and the subnet bit is the key.


Rather than me explain what is in the doc can you have a read of the "Glean Adjacencies" section -


http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml#express


So from this i think a glean entry cannot have punt next to it because the glean entry is only tied to a subnet. An individual host entry that falls with the subnet could have a punt next to it but not the subnet.


That is the way i understand it but am happy to admit i could be wrong. Finding very detailed docs on CEF in Cisco site has proved quite difficult. Would be nice if there was a document that had examples from routers/L3 switches for every possible type of adjacency.


If i get the chance i will do some tests and let you know later this week.


Jon

sarahr202 Sun, 12/14/2008 - 18:54

Thanks Jon for your help and reply. I undesratand my questions are way too weird, but then I think there is nothing wrong to find the answers that dont know.

In your reply, it was said:


"A glean adjacency is one where the host is directly connected but there is no mac-address information for it".


My undersyanding is adjacency table cotains

next hop(ip) mac address

I did not list the other fields for brevity.

Next hop means the host is directly connected.

If the mac address is missing, then we get glean adjacency or punt adjaency which is moot point atleast for now.

thanks a lot Jon! I would be waiting for your lab results.

sarahr202 Mon, 12/15/2008 - 05:05

I want to edit my reply where i said"Next hop means the host is directly connected"

Next hop means Router/Multilayer switch.

thanks a lot!

sarahr202 Sun, 12/14/2008 - 19:15

Thanks for your reply! I would go through that link with fresh mind and coffee !

sarahr202 Mon, 12/15/2008 - 06:53

Hi everybody!


http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml#express


The topology from the link is;

.1 .50

R,E2/0----------------H

10.1.1.0/24

Let me quote from the link:

"The four adjacency table entries built by default are:


10.1.1.0/24, version 17, attached, connected

0 packets, 0 bytes

via Ethernet2/0, 0 dependencies

valid glean adjacency

10.1.1.0/32, version 4, receive

10.1.1.1/32, version 3, receive

10.1.1.255/32, version 5, receive


Note there are four entries: three receives, and one glean. Each receive entry represents a broadcast address or an address configured on the router, while the glean entry represents the remainder of the address space on the attached network. If a packet is received for host 10.1.1.50, the router attempts to switch it, and finds it resolved to this glean adjacency. Cisco Express Forwarding then signals that an ARP cache entry is needed for 10.1.1.50, the ARP process sends an ARP packet, and the appropriate adjacency table entry is built from the new ARP cache information. After this step is complete, the adjacency table has an entry for 10.1.1.50.

The next packet the router receives destined for 10.1.1.50 is switched through this new adjacency."

Based on the above, my understanding is adjacency table can contain 10.1.1.50-mac-address which is not the next hop,rather it is the address of host .

I thought next hop means directly connected router .

My understanding is If a switch(L3)receives a packet say 1.1.1.1.

prefix next hop

1.1.1.0 192.192.192.192

( i did not list the other fields for brevity).

The fib is checked, the longest match is selected, For next hop to mac , Adjacency table is checked and mac address is read from the table.


I think it all comes down to one question:

Can a host be next-hop?

I think routers/L3 switches are next hop not hosts.

Any suggestion?

thanks a lot!

Any suggestion

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